1. Field of the Invention
The present invention relates to a technique for controlling vibration of a movable object in a precision apparatus, such as a semiconductor exposure apparatus, a machine tool, an OA (office automation) apparatus, or the like.
2. Description of the Related Art
Conventional techniques intended to simultaneously suppress a rigid vibration mode and an elastic vibration mode are described, for example, in “An apparatus for controlling a vertical-direction-air-spring-type vibration removing mount” disclosed in Japanese Patent Application Laid-Open (Kokai) No. 7-83276 (1995) (hereinafter termed “reference 1”), and in “A method for controlling multi-mode vibration in a vibration generation mount” disclosed in Japanese Patent Application Laid-Open (Kokai) No. 11-194066 (1999) (hereinafter termed “reference 2”).
In reference 1, four position deviations of four driving points of a vibration removing mount from a floor, and four acceleration values on the vibration removing mount are detected. A motion-mode extraction circuit for calculating position deviations and acceleration values for respective motion modes from respective signals representing these position deviations and acceleration values is prepared. A driving signal for each mode is calculated from the obtained position deviations and acceleration values for each motion mode. The calculated driving signal is converted into a driving signal for each air spring using a motion-mode distribution circuit, to drive the air spring. According to this control method, four degrees of freedom, i.e., three degrees of freedom for rigid vibration and one degree of freedom for elastic vibration, are controlled.
In reference 2, in order to control a large number of elastic vibration modes of a vibration generation mount, a method is proposed in which the vibration generation mount is approximated to a model of connection of a finite number of material particles, and elastic vibration is suppressed by feeding back the displacement, the velocity, and the like, of each material particle.
A conventional technique for suppressing elastic vibration of a movable body is described, for example, in “Magnetic disk apparatus” disclosed in Japanese Patent Application Laid-Open. (Kokai) No. 05-225734 (1993) (hereinafter termed “reference 3”). In this technique, piezoelectric elements for driving and detecting elastic vibration are provided at a spring arm of a magnetic disk, and a resistor connecting the piezoelectric elements and a control circuit for changing the value of the resistor are also provided. The value of the resistor is switched between “seek”, in which the arm is greatly rotated and moved, and “on-track”, in which the arm is slightly moved. Thus, elastic vibration of the spring arm is suppressed during seek, and the spring arm is used as an elastic member during on-track.
A method for suppressing elastic vibration of a beam, instead of a movable body, is described, for example, in “Detection and control of beam vibration using a piezoelectric film” (Nippon Kikai Gakkai Ronbunshu, C, Vol. 63, No. 615, hereinafter termed “reference 4). In this method, as in the above-described reference 3, elastic vibration of a beam is suppressed by bonding piezoelectric elements on both surfaces of a beam, amplifying the voltage of a piezoelectric element for detection, and inputting the amplified voltage to a piezoelectric element for driving.
In the method of reference 1, each of a position measuring device and an acceleration measuring device measures vibration (motion) as a result of synthesizing rigid vibration and elastic vibration. In order to separate elastic vibration and rigid vibration, a motion-mode extraction circuit is necessary. As a result, the configuration of circuitry becomes complicated. A simple square plate is assumed as the vibration removing mount described in reference 1, and a conversion matrix to be used by the above-described conversion circuits has a simple form. In an actual vibration removing mount, however, since other components are also present and vibration modes are complicated, it is not easy to separate motion modes. In reference 1, vibration in a rigid mode and vibration in an elastic mode are detected by the same detector. In the case of reference 1, vibration in the rigid mode has a frequency of several Hz, and vibration in the elastic mode has a frequency of several tens of Hz. Accordingly, in order to detect both of these vibrations, it is necessary to prepare a detector having a wide dynamic range.
In reference 2, vibration as a result of synthesizing elastic vibration and rigid vibration is also detected. In the method of reference 2, in order to realize desired rigid motion, it is also necessary to separate rigid vibration and elastic vibration according to some approach. In the method of reference 2, however, nothing is described with respect to this point. That is, no method for arbitrarily controlling rigid vibration while suppressing elastic vibration is described.
References 3 and 4 disclose methods for suppressing elastic vibration by detecting elastic vibration of an elastic member, and feeding back the measured value to driving means for elastic vibration. Accordingly, in these methods, attenuation characteristics are determined depending on how high the gain of a feedback loop can be made. In the method of reference 3, since no amplifier for supplying electric power is present, too excellent attenuation characteristics cannot be obtained. In the method of reference 4, there is a limitation in the gain of a realizable feedback loop, depending on the positions of piezoelectric elements bonded on an elastic member, the characteristics of the piezoelectric elements, and the like. Accordingly, there is a limitation in the obtained attenuation characteristics. As described above, there is a limitation due to insufficient stability in improvement of attenuation characteristics by feedback control, and required attenuation performance is not always obtained.